1. Field of the Invention
The present invention relates to an adjustable valve timing control apparatus for an engine.
2. Description of the Related Art
The intake valve and exhaust valve of an engine should be opened and closed at accurate times to efficiently accomplish the suction and exhaust in cylinders. The optimal valve timing varies according to the engine speed, engine load and the like. A variable valve timing mechanism which changes the phase of the intake-side cam shaft based on the low speed or high speed is employed to cope with such a variation.
Japanese Unexamined Patent Publication No. Sho 63-131808 discloses the above variable valve timing mechanism. According to this prior art, as shown in FIG. 7, a spool 103 is located between a housing 101, coupled to a pulley 100 which rotates in synchronism with the engine, and a valve-driving cam shaft 102. A spline 104 formed on the outer surface of the spool 103 is engaged with a spline 105 formed on the inner surface of the housing 101. A spline 106 formed on the inner surface of the spool 103 is engaged with a spline 107 formed on the outer surface of a housing 108. Therefore, the spool 103 transmits the rotation of the pulley 100 to the cam shaft 102 and moves in the axial direction of the cam shaft 102 with respect to the housing 101 and housing 108.
At least one of the engagement between the housing 101 and the spool 103 and the engagement between the spool 103 and the housing 108 is accomplished by a helical spline. The housing 101, spool 103, housing 108 and pulley 100 define a first pressure chamber 109 and a second pressure chamber 110 with the spool 103 in between. An electromagnetic oil control valve (hereinafter referred to as "OCV"), disposed in an oil passage which connects both pressure chambers 109 and 110 to an oil pump 111, is controlled by the duty cycle that is based on the running condition of the engine, thereby adjusting the hydraulic pressure to be supplied to the pressure chambers 109 and 110. Accordingly, the spool 103 is moved in the axial direction of the cam shaft 102 by the differential pressure between both pressure chambers 109 and 110. At the same time, the helical spline engagement of the spool 103 applies twisting force to the cam shaft 102 to advance or retard the cam shaft 102 with respect to the crankshaft by the pressures in the first pressure chamber 109 and the second pressure chamber 110. As a result, the valve opening and closing timings are adjusted.
To change the valve timing using this apparatus, the target valve timing to be controlled is computed based on the running condition of the engine. The feedback control of the OCV is executed so that the actual valve timing converges to the target valve timing. Further, when the actual valve timing coincides with the target valve timing, the OCV is controlled in the first duty cycle for holding the spool 103 at the position of coincidence. Consequently, the variable valve timing mechanism is maintained at the position where the actual valve timing matches with the target valve timing.
The aforementioned Japanese Unexamined Patent Publication No. Sho 63-131808 teaches nothing about the control for holding the spool 103 at the shifted position thereof when the spool 103 is moved, in order to retard the cam shaft 102 to its limit with respect to the crankshaft.
To control the target valve timing of the cam shaft 102 to the maximum lagged angle, for example, the OCV may be so controlled as to supply the operational oil only to the second pressure chamber 110. This control allows only the hydraulic pressure to be supplied to the second pressure chamber 110 to surely hold the spool 103 at the position in the first pressure chamber 109 at which it abuts on the housing 101.
But, this control does not supply the operational oil to the first pressure chamber 109 so that when the spool 103 is kept pressed against the housing 101, the operational oil in the oil passage that connects the first pressure chamber 109 to the oil pan completely escapes to the oil pan. Even if an attempt is then made to increase the pressure in the first pressure chamber 109 to shift the cam shaft 102 to the phase of the angle-advancing side, it would take time to fill the operational oil in the oil passage, thus resulting in a lower control response.
To overcome the aforementioned problem, the present applicant (assignee) has proposed a method of controlling the OCV according to a slightly-offset duty cycle when the spool is moved to a position for shifting the cam shaft 102 retarded to its limit with respect to the crankshaft. This control allows the operational oil to be supplied to the first pressure chamber 109 by the hydraulic pressure slightly lower than that in the second pressure chamber 110. It is thus possible to keep the spool 103 abutting on the housing 101 in the first pressure chamber 109. To move the spool 103 in the opposite direction, the OCV should be controlled to cancel the slight differential pressure between both pressure chambers 109 and 110. This control therefore has an excellent control response.
The above-described variable valve timing mechanism suffers a conventionally known and inherent problem such that noise is produced by the teeth of the spline coupling portions of the spool 103 and housing 101 and/or the spool 103 and housing 108 hitting one another due to the back lash. This noise becomes more prominent as the engine speed increases, and becomes uncomfortably large particularly when the acceleration pedal is thrust forward in the loadless state of the engine such as when a vehicle is stationary.
In consideration of the response of the cam shaft 102 with respect to the duty cycle for controlling the OCV, the above-discussed variable valve timing mechanism is so designed that the hydraulic pressure to be supplied to the first pressure chamber 109 stays slightly above the hydraulic pressure to be supplied to the second pressure chamber 110. In other words, the force that pushes the spool 103 against the housing 101 is small. The splines 104 and 106 of the spool 103 will not be pressed against the matching splines 105 and 107 with a large surface pressure. Therefore, this variable valve timing mechanism should still cope with the generation of the back-lash originated noise.
In the other variable valve timing mechanism proposed by the present applicant, the housing is fixed in this mechanism and faces the pulley with a slight clearance therebetween. As the spool moves, therefore, the housing 108 receives force from the spline coupling portions. Accordingly, the pulley moves to and hits the pulley. In addition to the back-lash originated noise, there may be produced a sound of abutment of the pulley against the sleeve as the spool moves to shift the phase of the cam shaft advanced or retarded with respect to the crankshaft.